1. Technical Field of the Invention
The present invention concerns a tire equipped with a nail-type force measurement device.
2. The Related Art
U.S. Pat. No. 5,864,056 discloses a nail-type force sensor for tires, wherein the shank of the sensor is a flexible blade fitted with two strain gauges mounted back to back and the head of the sensor comprises at least two rigid discs transverse to the shank, separated by a tube orientated in the direction of the shank.
The major disadvantage of this structure is its lack of sensitivity along the axis of the shank. It can only pick up the components of a force located in a plane transverse to the axis of the shank. Now, it is important to know all of the components of the force, so that with one and the same sensor a lack of grip, for example, can be detected.
A nail-type force sensor for tires is also known from patent application EP-A-O 937 615. This nail-type sensor has a rigid shank, one end of which is free while the other end is attached to a head in the form of a first, electrically conducting plate transverse to the shank. This first plate cooperates with other electrically conducting plates positioned opposite and separated by a dielectric material, to form condensers. The shank is designed to be inserted by its free end into the tread of the tire. When the shank is acted upon by a force corresponding to a stress undergone by the tire tread, it displaces the first plate with it as it moves, whereby the values of the capacities of the condensers so formed vary. This structure, in its turn, has the following disadvantages.
The sensor is a macroscopic device. By its very presence, it can modify the mechanical behavior of the tread whose forces it is intended to measure. The various parts of the sensor cannot be assembled by microelectronic techniques.
This sensor only operates with condensers that require proximity processing electronics.
The dielectric may have electrical and elastic properties that vary with environment, temperature and humidity. These variations must be taken into account when interpreting the measurements.
The object of the present invention is to produce a tire provided with a nail-type force sensor which does not have the drawbacks of the prior art, in particular lack of sensitivity along the axis of the shank and uncertainty concerning the information it supplies.
In accordance with the invention, a tire is equipped with a nail-type force sensor comprising a rigid shank intended to be acted upon by the force to be picked up, and a head that comprises an element attached to the shank and is designed to be deformed or stressed when the shank is moved. The deformable element comprises means to measure the deformation or stress, and has anchoring points which have a given position relative to the shank at rest and which essentially preserve that position when the shank moves.
The application of the force to the shank at a certain distance from the deformable element and the anchoring of the deformable element at several points make it possible to determine not only the force component in a plane transverse to the shank but also the force component along the axis of the shank.
The anchoring points may be discrete or continuous along the edge of the deformable element.
The deformable element may be a full membrane or it can comprise radial arms connected together at a central area where the shank is attached. In the latter configuration, the anchoring points are located at the ends of the arms.
Four essentially orthogonal arms can be used. If there are three arms, it is preferable to space them essentially at intervals of 120xc2x0 so as to obtain a certain symmetry around the axis of the shank.
The anchoring points are designed to cooperate with anchoring means, which can take the form, for example, of a cap, a frame or studs. The cap is particularly advantageous, notably because by fixing it on the anchoring points in an airtight way it contributes towards delimiting a hermetic cavity in which a controlled atmosphere can be maintained. The attachment between the cap and the anchoring points can incorporate at least one electrically conducting path.
The shank and the deformable element may be made on the basis of materials such as silicon, quartz, glass or a metal.
The measurement means can consist of at least three deformation or stress measurement devices distributed on the deformable element. Such measurement devices can be piezoelectric or piezoresistive gauges, condensers, or extensometric metallic gauges.
A particularly advantageous configuration is to make the deformable element of silicon and arrange the measurement devices along the  less than 110 greater than  crystal axes of the deformable element.
Other characteristics and advantages of the invention will emerge on reading the description below, illustrated by the attached figures and drawings. The description is presented with illustrative and not limiting intent.